1. Field of the Invention
The present invention generally relates to video coding apparatus, video decoding apparatus, video coding methods, and video decoding methods. More particularly, the present invention relates to a video coding apparatus that performs predictive coding of digital video signals, a video decoding apparatus that reproduces original motion images from a predictive-coded video bitstream produced by the video coding apparatus, and coding and decoding methods implemented in those video coding and decoding apparatus.
2. Description of the Related Art
In the field of digital motion picture coding techniques employing hybrid coding algorithms, H.261, MPEG-1 and MPEG-2, for example, are well-acknowledged international standards. H.261 has been formulated by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), while MPEG-1 and MPEG-2 have been defined by a committee named Moving Picture Experts Group under the auspices of the International Standards Organization (ISO). Hybrid coding algorithms are actually a combination of different coding techniques, which compresses video signals by reducing their spatial and temporal redundancies in the following sequence: (1) a source picture is divided into blocks of picture elements (pels or pixels), (2) motion compensation and orthogonal transformation (e.g., discrete cosine transform) are applied independently on each block of pels, and (3) transform coefficients are quantized and then compressed with entropy coding techniques.
When a rapid motion or a full scene transition has happened during a sequence of video frames, the above-described hybrid video coder may suffer from an overwhelming amount of coded frame data exceeding a certain standard level allowed for each frame. In such a situation, the video coder will forcibly reduce the amount of coded data in an attempt to regulate it at the standard level defined by the bandwidth of communication channels used. This attempt, however, can lead to extreme degradation in video image quality and frequent frame drops, thus resulting in unacceptably poor pictures when reconstructed at the receiving ends.
To solve the above problems, a video coding system is disclosed in the Japanese Patent Application Laid-open Publication No. 7-95566 (1995), for instance. In this proposed system, the video coder reduces the resolution of input frame signals to regulate the amount of coded frame data when a full scene change or a rapid motion has been observed in the middle of a sequence of video frames. More specifically, the video coder divides a picture into blocks and estimates motion vectors of the individual blocks as parameters for motion-compensated frame prediction. It then calculates the average magnitude of the estimated motion vectors over the entire picture. Based on the average magnitude of motions, the proposed video coder determines at what picture resolution the present frame should be encoded. This decision affects the format of both source and reference pictures; they have to be converted to the determined resolution before being subjected to the coding process.
In addition to the above, another Japanese Patent Application Laid-open Publication No. 63-155896 (1988) proposes a similar video coding system which encodes video signals after converting both source and reference pictures to a lower resolution.
The reference picture serves as the basis for predictive coding/decoding of future frames. It should be noted, however, that the above-described conventional video coding systems subsample the entire area of this reference picture, when the picture resolution mode has changed from high resolution mode to low resolution mode. While efficiently reducing the coded data size, this picture format conversion causes an adverse side effect. In a videoconference, for example, most background portions of video images do not change with time, and such static portions never contribute to temporal increase in the amount of video information. Therefore, the reduction of picture resolution is unnecessary as far as the background portions are concerned. The conventional video coding devices, however, destroy the clarity of static background images by reducing the picture resolution, thus causing an intense degradation in visual quality.